This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section
Network administration and management entails configuration and monitoring of virtually all the network elements, including transmitters, receivers, switches and add-drop multiplexers. The accomplishment of this task requires exchange of service information along the network.
Transport networks require the support of critical functionalities as Operations, Administration and Management (OAM). In an optical network it may be beneficial transmitting auxiliary service information associated with individual channels or channel groups. This may include the used wavelengths, the adopted modulation format, the Forward Error Correction (FEC) type, the accumulated chromatic dispersion, routing information, source and destination identifiers, alarms and, possibly, parasitic low-rate channels.
OAM information should be propagated reliably by using the lowest possible amount of network resources and should be ideally accessible and modifiable at any network element.
In order to simplify monitoring and management of large networks, read and write access to the service information should be possible both for single channels and for channel groups and channel-specific information should be routed congruently with the associated channel.
A large number of methods to transmit OAM information out-of-band are known from the prior art. Cited for example is IETF RFC 3945, “Generalized Multi-Protocol Label Switching (GMPLS) Architecture,” October 2004, where, according to the Generalized Multi-Protocol Label Switching (GMPLS) method, OAM information is transmitted on a dedicated wavelength or over a separated network. This approach, however, does not fulfill the congruence requirement for OAM. Explicit measures are required to enforce co-routing with the associated channels, which results into error prone solutions. Moreover, the use of additional resources for OAM signalling is possibly associated with inefficient implementations.
According to an alternative known method, cited in IETF RFC 5654, “Requirements of an MPLS Transport Profile”, September 2009, signalling information can be multiplexed with the data in the digital domain through the implementation of the Generic Associated Channel (G-ACh). With this approach, however, both read and write access to the service information require a termination of the data channel, which, in the case of optical communications, implies expensive electro-optical conversion with a full-fledged receiver.
The use of Sub-Carrier Multiplexing (SCM) for the transmission of control channels is discussed in the document: S. F. Su, and R. Olshansky, “Performance of Multiple Access WDM Networks with Subcarrier Multiplexed Control Channels,” J. Lightwave Technol., vol. 11, no. 5/6, pp. 1028-1033, 1993., whose authors consider service signalling for the control of a multiple access Wavelength Division Multiplexing (WDM) network. According to the cited document, the control channels are frequency-multiplexed with the data channel and a single wavelength can accommodate, besides the data channel, multiple sub-carriers. Every SCM receiver scans only one dedicated sub-carrier and the SCM transmitters must tune their transmit frequency according to the addressed SCM receiver. At each SCM receiver all optical wavelengths reach the photodiode; the SCM receiver extracts only its own associated subcarrier out of the resulting electrical signal. Unfortunately, with this solution each service receiver can communicate only with a single service transmitter at a time, which prevents the implementation of genuine OAM. Moreover, the number of reserved sub-carriers needs to grow with the network size, which results into complex SCM transceivers and inefficient resource exploitation.
A method and an apparatus for encoding, transmitting and decoding labels in an optical packet network are introduced in the U.S. Pat. No. 7,512,342 B1 (M. D. Feuer and V. A. Vaishampayan, “Digital Encoding of Labels for Optical Packet Networks,”) and in the U.S. Pat. No. 7,630,636 B1 (M. D. Feuer and V. A. Vaishampayan, “Optical Swapping of Digitally-Encoded Optical Labels”). In the cited documents the payload data are encoded through a Complementary Constant Weight Code (CCWC) of rate K/N, which increases the line rate and provides the necessary room for the transmission of controlling information in the form of packet labels. In the known method, the CCWC associates each block of K payload bits with two codewords of N bits having different binary weights. The transmitter chooses between the low-weight and the high-weight codewords according to the value of the packet label. Every K payload bits, the transmitter can convey an additional label bit by modulating the binary weight of the transmitted stream. Since in the case of conventional On-Off Keying (OOK) the binary weight is proportional to the transmit power, the labels can be recovered by detecting the average power of the received signal with a low-speed photodiode without terminating the data channel. For WDM networks, code division multiplexing can be used to separate labels associated with multiple wavelengths.
Unfortunately, the use of the Complementary Constant Weight Code (CCWC) increases the line rate and consumes part of the available overhead that can be allocated to Forward Error Correction (FEC).
Moreover, since the CCWC is not systematic, the wrong detection of a single CCWC codeword results into multiple bit errors. This error multiplication has a detrimental effect on the performance of the subsequent FEC decoder. In particular the design of the CCWC is heavily constrained, because a short code length N results into a significant overhead, whereas a long code length results into considerable error multiplication.
Additionally, the approach is not “transparent”, in the sense that unaware receivers are not able to recover the payload data that have undergone CCWC encoding.
Finally, the method does not foresee label stacking along the optical network nor read and write access per wavelength group.
The problem to be solved is to avoid the disadvantages mentioned above and in particular to provide a method which allows the implementation of genuine optical Operation, Administration and Maintenance (OAM) for optical communication networks in which a single service transmitter may transmit the same service information on multiple optical signals at the same time.
A cost efficient technique is needed that allows read and write access to service information without termination of the data channel and transmission of per-group service information.